Added capability to use EXIF rotation metadata, using the imagemagick -a...

...uto-orient option.

As part of this, streamlined create_constrained_square_crop() to use the IM -extent option rather than in-house code (see http://www.imagemagick.org/script/command-line-options.php?#extent)

I have tested the imagemagick options on my local computer. They require ImageMagick >- 6.2.4, but otherwise work fine with EXIF rotated images.